1. Field of the Invention
The present invention relates generally to an air/fuel ratio control system for an internal combustion engine for performing .lambda. control or feedback control in order to maintain an air/fuel ratio close to a stoichiometric value. More specifically, the invention relates to an air/fuel ratio system which can perform .lambda. control when the engine is driven in an idling state.
2. Description of the Background Art
As is well known, air/fuel ratio control for an air/fuel mixture to be introduced into an engine combustion chamber is performed by modifying a basic fuel delivery amount Tp, such as the basic fuel injection amount, which is generally derived on the basis of engine revolution speed and engine load represented by an intake air flow rate, for example, by utilizing a correction coefficient derived on the basis of an oxygen concentration dependent control parameter derived by monitoring oxygen concentration in the exhaust gas. The correction coefficient, which is variable depending on oxygen concentration in the exhaust gas, will be hereinafter referred to as the ".alpha. correction coefficient". The .alpha. correction coefficient is generally derived through a PI (proportional-integral) control process. The .alpha. correction coefficient consists of a lean mixture proportional component P.sub.L which is used when the air/fuel ratio varies from rich to lean across a stoichiometric value, a rich mixture proportional component P.sub.R which is used when the air/fuel ratio varies from lean to rich across the stoichiometric value, and a lean mixture integral component I.sub.L which is used while the air/fuel mixture is held lean, and a rich mixture integral component I.sub.R which is used while the air/fuel mixture is held rich. The integral components are derived by integrating an integral constant over a period while the air/fuel mixture is maintained rich or lean. In a practical process, the .alpha. correction coefficient is derived on the basis of the deviation of the air/fuel ratio from the stoichiometric value. Every time the air/fuel ratio varies from lean to rich or from rich to lean across the stoichiometric value, the air fuel ratio for the air/fuel mixture is controlled depending upon the proportional component P.sub.L or P.sub.R.
In the case where the engine is driven in the idling state, the idling engine speed tends to change unsuitably even if the air/fuel ratio changes only slightly due to the .alpha. correction coefficient, and the engine speed becomes unstable in the idling state. Therefore, when the air/fuel ratio nearly approaches the stoichiometric value in the idling state, it is preferable to fix the .alpha. correction coefficient in order to stabilize the engine idling speed.
However, there are cases in which the air/fuel ratio deviates substantially from the stoichiometric value and is held lean due to dispersion or a change over time in the characteristics of a fuel injection valve or so forth. In this case, if .lambda. control is not performed, engine stall may occur.
In order to eliminate the aforementioned disadvantage, an air/fuel ratio control system which is designed to fix the .alpha. correction coefficient for a predetermined period of time, or a predetermined number of cycles, after the engine enters an idling condition, is proposed in Japanese Patent First (unexamined) Publication (Tokkai Sho.) No. 56-143325. In this system, in order to suitably control the air/fuel ratio even if the air/fuel ratio deviates substantially from the stoichiometric value and is held lean or rich, .lambda. control is performed for a predetermined period of time T after the engine starts to be driven in the idling state. The average of the .alpha. correction coefficients for the predetermined period of time T is fixed to the modified .alpha. correction coefficient which is used for deriving the fuel injection amount Ti, so as to stabilize engine idling speed.
However, the aforementioned system can not suitably control the air/fuel ratio, for example, when the engine coolant temperature is relatively low and a rich air/fuel mixture is required or so forth. That is, since the modified .alpha. correction coefficient is fixed to the average value of the .alpha. correction coefficients derived for the predetermined period of time, independent of the engine coolant temperature, an air/fuel mixture having the required air/fuel ratio can not be supplied to the engine when the engine coolant temperature is low.
Therefore, in the aforementioned system, it is not possible to prevent combustion conditions from worsening and/or avoid an unstable engine idling speed, when the engine coolant temperature is low.